1. Field of the Invention
This invention relates to a method of producing a semiconductor laser module for use in the optical fiber communication, and more particularly, relates to a method of producing a semiconductor laser module capable of raising coupling efficiency up and reducing the time for aligning drastically.
2. Description of the Prior Art
FIG. 1 shows a standard structure of an ordinary semiconductor laser module. A package 2 has a semiconductor light emitting device chip 1 and a photodiode 3 for monitoring. There is a pole 6 standing perpendicular to the package 2 near the center of the package 2. The light emitting device chip 1 is fixed to a side surface of the pole 6 via a submount 8. An upper surface of the package 2 is covered with a cap 7 having a window and a lens in the window. In the case of the light emitting device being a laser diode, the monitoring photodiode 3 is fixed to the center part of the package 2 via another submount 9 so as to detect the light launched from the back end of the laser diode chip 1. An end surface 10 of an optical fiber 5 is slantingly polished for preventing reflection light from going back to the laser chip 1. Such returning light would lead to an instable condition of the laser 1. The slanting end 10 is able to prevent noises caused by the instable laser condition.
The light going from the laser 1 outward (forward) is converged by the ball lens 4, and enters a core of the end surface 10 of the optical fiber 5. The light going from the laser 1 to inward (backward) enters the monitoring photodiode 3, and the power of light is constantly supervised. A core axis line of the fiber 5, the center of the lens 4, the center of an emitting part of the laser 1 and the center of the photodiode chip 3 are aligned on the same straight line, that is, are aligned on the same optical axis.
FIG. 2 shows an example of the light emitting device modules practically used. A package 2 shown in FIG. 2 has the same inner structure as that of FIG. 1. FIG. 2 shows a module where parts for fixing an optical fiber 5 are supplied to the semiconductor laser shown by FIG. 1. A cylindrical sleeve 14 is placed on the package 2 and is fixed thereto. A cylindrical ferrule 15 holds an end part of the optical fiber 5. The ferrule 15 is inserted into a cylindrical ferrule holder 16. A bottom surface of the ferrule holder 16 is fixed to a top surface of the sleeve 14. The edge 24 of the ferrule 15 is also slantingly cut together with the fiber. Here, Z-axis is defined as the optical axis direction, and XY plane is defined as the plane perpendicular to Z-axis.
FIG. 2 shows a fixing type (pig-tail type) module in which the end of the optical fiber is fixed, but it is possible to produce another type of module in which an optical fiber can be freely attached or removed. This is, for example, a receptacle type module which is shown by FIG. 3. A dummy fiber 17 is maintained by a ferrule 18 for dummy fiber, and the ferrule 18 is supported by a holder 19. Further, the holder 19 is retained by a cylindrical housing 20. The alignment is carried out among a laser 1, a lens 4, and the dummy fiber 17 as they align on the same axis line.
There are four different alignments for producing a semiconductor laser module, which will be explained.
Alignment 1
First alignment is done for fixing a cap 7 with a lens to a package 2 at the most suitable position. The cap 7 is moved parallel in XY plane, as monitoring the power of light launched from the lens in the direction perpendicular to the lens middle plane. The point which maximizes the perpendicular light from the lens is researched, and the cap is settled on the package at the maximum point. Hence, this alignment is the operation of making the central axis line of the lens accord with the emission point of a laser in XY plane. The positioning of lens requires optical means. The laser should be fixed at the center S of the package as an aim position, but it is actually difficult to dispose the laser exactly at the center S. The position of a lens sometimes deviates from the center S of the package. This alignment is required for bringing the center of the lens to the place just above the laser in conventional modules.
Alignment 2
Second alignment is done for settling a ferrule 15 to a holder 16. The holder 16 and the ferrule 15 are relatively moved axially in z direction on their cylindrical surfaces. This alignment is ordinarily practiced, but is not concerned with the improvements by the present invention. Therefore, the detailed explanation may be omitted hereinafter.
Alignment 3
Third alignment is done for fixing the holder 16 to a sleeve 14. The holder 16 and the sleeve 14 are relatively moved parallel in G surfaces where the bottom surface of the holder 16 is fitted to the upper surface of the sleeve in XY plane. This alignment is the operation for bringing the end 10 of the optical fiber to an image point of the laser by the lens.
Alignment 4
Forth alignment is the rotation of the ferrule 15 around Z-axis on the cylindrical surfaces of the holder 16 and the ferrule 15. This has been hardly necessary for the conventional alignment which aligns a laser, a lens and an optical fiber on the same axis line. This becomes, however, necessary for an alignment of a new module which does not arrange a laser, a lens and an optical fiber on the same axial line. This rotation alignment would be indispensable for the new module having different axes for a laser, a lens and an optical fiber. The rotation alignment is so difficult that it spends a lot of time, which raises the cost of producing modules. The present invention aims at improvements of the forth alignment.
There have been no problems for the prior semiconductor laser modules having the above mentioned structures in their basic performance. According to the enhancement of optical communication applications, low cost and high output power of light (which is the power coupled with an optical fiber, and is briefly written as Pf) have been required for laser modules. A contrivance should be done for satisfying two requirements, that is, high output power and low cost which oppose each other by nature.
For example, Pf=0.1 mW.about.0.2 mW is sufficient for light power in the case of short-distance transmission of prior optical communication. High output power ranging from 0.5 mW to 1 mW is required for light power in fibers in the case of medium-distance transmission in near future. There is a possibility of contriving individual parts, e.g. lenses, chips, optical systems, and so on in order to enhance the output power Pf.
Adopting an aspherical lens is one method of raising Pf of the modules shown by FIG. 2 and FIG. 3, because an aspherical lens is superior to a spherical lens in aberration and in focusing ability. Such an aspherical lens is, however, too expensive to produce low-priced modules. Two-lens type module where two lenses are used is another method of raising Pf. This method can also enhance both the focusing ability and input power Pf, but can not solve the problem of high production cost yet.
Another method is to produce a semiconductor laser chip endowed with high output power Pf. However, the present invention does not adopt this method, but intends to use conventional semiconductor chips.
A primary factor of decreasing Pf is attributed to an optical fiber end surface that is slantingly cut. Even if light launched from a laser just hits the center of an optical fiber, a large power of light is refracted obliquely by the slanting end of the optical fiber, and is excluded from a core. In other words, the path of light emitted from the core of a single mode fiber is not on an extension of the fiber axis line but has a certain slanting angle to the extension of the fiber axis line. However, the laser lies on the extension of the fiber axis line. Hence, the center of the laser does not exist on the optical path of the light going out of the fiber. Such a geometrical mismatching makes Pf decrease.
Some improvements have been proposed so as to eliminate the geometrical mismatching and increase Pf thereby.
These improvements will be explained as follows.
1 Japanese Laid Open Patent Application No.1-292877 (292877/1989)
A lens and a semiconductor laser are installed on an optical path of the light emitted from a slanting cut surface of an optical fiber. Hence, the positions at which the lens and the semiconductor laser are disposed deviate from the extension of the optical fiber axis line. As a result, when the light emitted from the laser strikes and is refracted on the slanting cut surface in compliance with geometric optics, the refracted light proceeds exactly along the optical fiber axis line. Therefore, Pf increases. In this application, however, there is no explanation with respect to the fixing means of the optical fiber, the lens and the semiconductor laser. Further, this application never touched upon the alignments among an optical fiber, a lens and a laser. It is considered that the alignment should be extremely difficult, because the lens and the laser are not on the extension of the optical fiber axis line, but there is no explanation regarding the alignment and the difficulty of alignment.
2 Japanese Patent Publication No.5-56483 (56483/1993)
This prior document discloses a laser module having a more asymmetrical structure. A ferrule sustaining an end of an optical fiber is inclined to an axis line of a ferrule holder. A cylindrical part of the ferrule is put into a slanting hole that is bored in the ferrule holder, and is fixed thereto. Hence, the ferrule axis line, in other word, the optical fiber axis line is leaned to a package. When the laser light hits an optical fiber end, is refracted thereon, and goes into the optical fiber, the proceeding direction of light coincides with the optical fiber axis line. Whereby, Pf increases. It is, however, too difficult to make such a slanting ferrule holder, and further this method upheaves the cost of producing the ferrule holder. Furthermore, this laser module has difficulty in handling, because the optical fiber inclines to the holder.
The present invention gives more consideration to the proposal of the prior document 1, throws light on problems, and attempts to give a solution of the problems. What the present invention aims at and solves are explained by referring to FIG. 4 and FIG. 5. The relative positions of a laser G, a lens L and an optical fiber J differ from conventional ones. In FIG. 4, an optical fiber optical axis, a lens optical axis and a semiconductor laser optical axis are in parallel with each other but are not on the same common line. Here, there are discrepancies in three optical axes in XY plane. The semiconductor laser, the lens and the optical fiber are positioned on a slanting line in order. FIG. 4 is drawn by taking the optical axis of the lens, i.e. SHT as a standard. The semiconductor laser is positioned on the left side of the lens, and the axial distance between the semiconductor laser G and the lens L is denoted by "a". "P" is an emission point of the semiconductor laser G. The optical axis PN of the laser G deviates by x from the optical axis SHT of the lens.
There is an end of the optical fiber on the right side of the lens L. The axial distance b between the lens L and the fiber J is b=ma, where m is a magnifying power of the lens. Q is the center of the end of the optical fiber. The center P of the laser, the center H of the lens and the center Q of the optical fiber are on a straight line. The vertical distance x between the optical fiber axis QR and the lens axis ST is denoted by X=mx. There is a vertical gap of x between the laser axis PN and the lens axis ST, and there is a vertical gap of mx between the lens axis ST and the fiber axis QR. The light started from P is converged by the lens L, and forms an image of the laser on the end Q of the optical fiber. A beam of the light going out of the laser leads to the optical fiber J, passing through an optical path of PKQ. Another advancing beam parallel to the optical axis arrives at the center point Q of the optical fiber, tracing another optical path of PMQ. Intersection F of two lines, i.e. MQ and ST is distanced by a focal length f from the lens center H. Furthermore, another beam going out of P passes through the center H of the lens, and reaches Q, tracing a straight optical path of PHQ.
.theta. denotes a slantingly polished angle of the fiber end surface. In ordinary cases of on-axis incidence of light, an incident angle is equal to .theta., but in this case, the light enters the fiber end surface at a slanting angle of .alpha. to the axial line. FIG. 5 shows the directions of rays near the end of the optical fiber. An angle of .theta. is formed between the fiber axis line QR and the normal line n standing up on the fiber end surface. The light from the laser ordinarily enters along the fiber axis UQ. In this case, the laser light goes into the fiber at an incident angle of a to the fiber axis UQ. There exists a condition in order that the laser light MQ is refracted on the fiber end surface Q, and progresses along the axis line of QR. The condition will be shown by the following equation. EQU n.sub.1 sin (.alpha.+.theta.)=n.sub.2 sin .theta. (1)
Where n.sub.1 is the refractive index of air, n.sub.2 is the refractive index of the fiber core, .alpha. is the slanting beam angle, and .theta. is the slanting cut angle.
When Eq.(1) is satisfied, the light entering the optical fiber and propagating in the fiber is maximized. Hence, when the following equation is valid, Pf is maximized. EQU .alpha.=sin.sup.-1 (n.sub.2 sin .theta./n.sub.1)-.theta. (2)
The fiber axis QR and the laser axis SN are deviated from the lens axis SHT. It is known that the entering light at an oblique angle of .alpha. to the fiber axis is effective in increasing Pf. The prior documents 1 and 2 disclose nothing about how to stabilize the optical parts with axis deviation without losing industrial utilities nor how to suppress an increase of the production cost, and in particular, how to solve the difficulty of alignments.
In the concrete, since the positions where semiconductor laser chips (LD chips) are mounted are dispersed on packages, an operator should carry out the alignments for seeking the optimum point by moving parallel the fiber end in XY plane vertical to the fiber axis (z-axis) and by rotating the fiber end around the fiber axis (z-axis) at the same time. It may be, of course possible to do such complicate alignments, but it would take a plenty of time to finish the rotation alignment. Therefore, the production of modules would require a plenty of time and a great expenditure of money. Off-axis type semiconductor laser modules shown by FIG. 4 and FIG. 5 have never been produced nor utilized yet due to the difficulties above mentioned.